Falling Stars Cloud — the Cluster D capstone

Learning Goals 3 min

By the end of this lesson you will be able to:

1. Build a spawner that uses create clone of (myself v) on a timer to make new stars appear at random x positions along the top of the Stage.
2. Write a per-clone fall script under when I start as a clone that gives each star its own random speed and fades it as it falls using change [ghost v] effect by ().
3. Use a (count) variable as a population cap — only spawn when fewer than 50 clones exist, and decrement on delete this clone.

Warm-Up — predict the snowstorm 7 min

Aisyah is trying to make a "snow" effect using one Snowflake sprite and clones. She writes this on the original sprite:

when flag clicked
forever
  create clone of (myself v)
end
        

And under when I start as a clone, each snowflake does nothing — no movement, no hide, nothing. She clicks the flag. Predict — what happens?

This whole lesson is the Cluster D capstone — every cloning idea you've met (spawn-on-timer, when-I-start-as-a-clone, per-clone variables, delete this clone, counting clones) shows up in one beautiful effect.

New Concept — the spawn / live / die pattern 15 min

Every clone effect in Scratch — bullets, raindrops, sparks, enemies, falling stars — follows the same three-part pattern. Once you can name the three parts, you can build any of them.

Part 1 — the spawner (original sprite)

The original sprite (the one you drag onto the Stage) doesn't fall. Its job is to hide itself, then run a forever loop that waits a moment and then calls create clone of (myself v). The wait sets the spawn rate.

when flag clicked
hide
set [count v] to (0)
forever
  wait (0.3) seconds
  if <(count) < (50)> then
    create clone of (myself v)
  end
end
        

Part 2 — the per-clone life script

Under when I start as a clone, each new clone runs its own copy of a script. This is where it picks its random x, its random speed, shows itself, and falls.

Two things to notice about per-clone scripts:

• Every clone has its own copy of every variable if you make those variables "For this sprite only". So fall as a sprite-only variable means each star can have a different speed.
• The change [count v] by (1) at the very top is what keeps the cap honest — it tells the spawner "I exist, count me".

Part 3 — the death (delete this clone)

Clones that never die pile up forever. The fix is one block at the bottom of the per-clone script: delete this clone. Right before it, decrement the count, otherwise the count drifts upward and eventually freezes the spawner.

change [count v] by (-1)
delete this clone
        

The fade — change ghost effect by 3

The ghost effect is a number from 0 (fully visible) to 100 (invisible). Inside the fall loop, change [ghost v] effect by (3) nudges the star a little more transparent every frame. By the time it reaches the bottom, it's almost gone — a soft, atmospheric fade.

Worked Project — build the cloud in ten steps 15 min

Open Scratch. Delete the cat. Choose a dark backdrop — Stars or Space from the library. Add the Star sprite from the library (or paint your own — a small yellow blob is fine).

Step 1 — Make the variables

From Variables, click Make a Variable. Name it count and choose For all sprites. Untick its Stage checkbox (we don't want to see it on the final project). Then make another variable, name it fall, and choose For this sprite only — this is critical.

Step 2 — Shrink the star

Select the star sprite and set its size to about 30. From Looks: set size to (30) %. (You can drag this block in once and run it to set the costume size, then throw it away.)

Step 3 — Build the spawner

On the original star sprite, build:

when flag clicked
hide
set [count v] to (0)
forever
  wait (0.3) seconds
  if <(count) < (50)> then
    create clone of (myself v)
  end
end
        

Step 4 — Start the clone's life

Underneath, drag in when I start as a clone. This hat fires once for every new clone, on that clone's body.

Step 5 — Count, then position

The first thing each new clone does is bump the count and jump to a random spot along the top of the Stage:

when I start as a clone
change [count v] by (1)
go to x: (pick random (-220) to (220)) y: (180)
set [fall v] to (pick random (2) to (5))
set [ghost v] effect to (0)
show
        

Step 6 — Fall and fade

Add the fall loop directly below — a repeat until that runs until the star drops off the bottom of the Stage:

repeat until <(y position) < (-180)>
  change y by ((0) - (fall))
  change [ghost v] effect by (3)
end
        

Step 7 — Die properly

The last two blocks of the per-clone script. Decrement first, then delete:

change [count v] by (-1)
delete this clone
        

Step 8 — Click the flag

You should see a steady, gentle drift of stars from top to bottom, each one fading as it falls. Around the 15-second mark, you'll have roughly 50 stars on screen and the spawner pauses — exactly what we want.

Step 9 — Polish

Try swapping the wait time (0.3 → 0.1 for a blizzard, → 1 for a gentle drizzle), the cap (50 → 100 for chaos), and the fade rate (3 → 1 for slow fade, → 8 for sudden fade). Add set size to (pick random (15) to (50)) % inside the clone hat for varied depths.

Step 10 — The full assembled project

All on one sprite — the Star. Two scripts total.

when flag clicked
hide
set [count v] to (0)
forever
  wait (0.3) seconds
  if <(count) < (50)> then
    create clone of (myself v)
  end
end

when I start as a clone
change [count v] by (1)
go to x: (pick random (-220) to (220)) y: (180)
set [fall v] to (pick random (2) to (5))
set [ghost v] effect to (0)
show
repeat until <(y position) < (-180)>
  change y by ((0) - (fall))
  change [ghost v] effect by (3)
end
change [count v] by (-1)
delete this clone
        

What you just built: the exact same pattern that powers raindrops in a rain effect, bullets in a shooter, sparks in a fireworks demo, and falling petals in a sakura scene. The "thing" changes — the pattern stays.

Try It Yourself — three remixes 12 min

set [fall v] to (pick random (5) to (10))
          

when I start as a clone
change [count v] by (1)
go to x: (pick random (-220) to (220)) y: (180)
set [fall v] to (pick random (2) to (5))
set size to (pick random (15) to (50)) %
set [ghost v] effect to (0)
show
          

when I start as a clone
change [count v] by (1)
go to x: (pick random (-220) to (220)) y: (180)
set [fall v] to (pick random (2) to (5))
set [spin v] to (pick random (-5) to (5))
set [ghost v] effect to (0)
show
repeat until <(y position) < (-180)>
  change y by ((0) - (fall))
  turn cw (spin) degrees
  change [ghost v] effect by (3)
end
change [count v] by (-1)
delete this clone
          

Mini-Challenge — Daniel's mystery freeze 5 min

"The spawner that quit early"

Daniel built the falling stars project, but his spawner stops creating new stars after about 30 seconds — the screen empties and stays empty. The stars he did spawn fall and disappear normally. He shows you his per-clone script:

when I start as a clone
change [count v] by (1)
go to x: (pick random (-220) to (220)) y: (180)
set [fall v] to (pick random (2) to (5))
set [ghost v] effect to (0)
show
repeat until <(y position) < (-180)>
  change y by ((0) - (fall))
  change [ghost v] effect by (3)
end
delete this clone
change [count v] by (-1)
        

What's gone wrong? Why does the spawner go silent after a while?

change [count v] by (-1)
delete this clone
          

Recap 3 min

You shipped the Cluster D capstone. A single sprite, two scripts, eighteen blocks, and an effect that genuinely looks like a piece of generative art. You used every idea from this cluster: create clone of (myself v) on a timer, when I start as a clone as a per-clone life script, sprite-only variables so each clone keeps its own fall speed, a (count) variable as a population cap, and delete this clone with the decrement before it. Spawn, live, die, count. That's it. From here, the same pattern builds bullets, raindrops, enemies, sparks — anything that appears, behaves, and goes away.

Spawner

The original sprite whose only job is to hide itself and call create clone of (myself v) on a timer. It doesn't fall, fade, or do anything visible — the clones do all the work.

Per-clone variable

A variable made with the "For this sprite only" option. Every clone gets its own private copy. That's how each falling star can have a different fall speed without overwriting its neighbours.

Population cap

A maximum number of live clones, enforced by an if <> then around the create clone call. Protects against Scratch's hidden 300-clone hard limit and keeps the project smooth.

Ghost effect

A Looks effect from 0 (fully visible) to 100 (invisible). change [ghost v] effect by () nudges it up by a fixed amount every frame, producing a soft fade-out.

Decrement-before-delete

The rule that any clean-up — counting, broadcasting, saving — must happen before the delete this clone block, because delete halts the script instantly.

Homework 2 min

Reskin the cloud. Take your finished Falling Stars project and turn it into something else by changing the costume and a couple of numbers. Keep the spawn / live / die pattern intact.

1. Pick a theme: rain, snow, sakura petals, falling leaves, glitter, or your own idea. Replace the star costume with a sprite that fits the theme. (Library or your own paint.)
2. Tune three numbers to match the feel: the spawn wait () seconds, the fall range, and the fade rate (the number inside change [ghost v] effect by ()). Snow is slow. Rain is fast and barely fades. Petals drift.
3. Add one new sprite-only twist of your own — size, rotation, colour effect, costume switching. Anything per-clone.

Save as HW-L3-25-Cloud-Reskin.sb3.

Bring back next class:

• The .sb3 file with your reskin.
• One sentence: "My count cap is set to ___ because ___." (No wrong answers — just explain your reasoning.)

Heads up for next class: SCR-L03-26 kicks off Cluster E — Game State Machines. We meet the idea that a real game has phases (title screen, playing, paused, game over) and that one variable can control all of them. No live coding next class — pure concept. Bring brain.